The present invention relates to a multifocal spectacle lens and producing methods thereof.
Conventionally, multifocal spectacle lenses have been widely used to compensate for age-related decrease in amplitude of accommodation of eye. The multifocal spectacle lens includes a lens having a discontinuously changing refractive power such as a bifocal lens and a trifocal lens, and a progressive-power spectacle lens having a continuously changing refractive power.
In general, an upper area of a spectacle lens is used for distance vision and a lower area of the spectacle lens is used for near vision. Therefore, the conventional multifocal spectacle lens has a distance portion having dioptric power for distance vision at an upper area of the lens and a near portion having a dioptic power for near vision at a lower area of the lens.
However, in a particular case, it is required that the upper area of the spectacle lens is used for near vision or middle distance vision. For example, a pilot is required to stare into the distance through a center area of the field of view and to see, through an upper area or a lower area of the field of view, cockpit gauges, cockpit indications and flight controls, as well as manuals and printouts provided by the cockpit gauges.
That is, the pilot is required to use the upper area and the lower area of the spectacle lens for near vision or middle distance vision and to use the center area of the lens for distance vision.
Japanese provisional publication No. SYO 62-30216 discloses a mulitifocal spectacle lens which is configured to satisfy the above-mentioned requirement. Also, FIGS. 30 and 31 show other examples of conventional mulitifocal spectacle lenses L5 and L6 designed to satisfy the above mentioned requirement. Each of the multifocal spectacle lenses shown in FIGS. 30 and 31 and the publication is configured to have a multifocal surface or a progressive-power surface on a front surface thereof.
The lens L5 shown in FIG. 30 has a bifocal surface at an upper area of the front surface and a progressive-power surface at a lower area of the front surface of the lens.
The lens L6 shown in FIG. 31 is a so-called double D type trifocal lens. That is, the lens L6 has segments, each of which has a form of a letter D, arranged in the upper area of the front surface and in the lower area of the front surface of the lens, respectively.
The back surface of the conventional multifocal spectacle lens is formed as a spherical surface or a toric surface.
In general, the conventional multifocal spectacle lenses are produced as follows. Initially, vertex power (e.g., spherical power and cylindrical power) is classified into a predetermined number of classes.
Semifinished lens blanks are stockpiled for each of the classes. The semifinished lens blank of the conventional multifocal spectacle lens L5 is made of resin and a front surface of the lens L5 is molded. The semifinished lens blank of the conventional multifocal spectacle lens L6 is made of resin or glass. When the semifinished lens blank of the lens L6 is made of resin, a front surface of the lens L6 is molded. When the semifinished lens blank of the lens L6 is made of glass, a segment whose refractive index is different from that of a main lens L6 is bonded to the main lens by fusion splicing.
It should be noted that each semifinished lens blank has a predetermined value of spherical power and a predetermined value of cylindrical power within the corresponding class. In general, the predetermined value of the power (spherical power or cylindrical power) equals to a center value within a range of the power of the corresponding class.
One of the classes is selected according to a lens specification (i.e., a customer's specification). Then, a back surface (i.e., a spherical surface or a toric surface) of the semifinished lens blank belonging to the selected class is processed so as to meet a required dioptric power (i.e., the customer's specification).
It should be appreciated that, occurrence of variations in optical performance of a finished lens depends on a difference between the required power and the predetermined power of the selected semifinished lens blank.
That's because the semifinished lens blank is designed to exhibit the most effective optical performance at the center value within the range of power of the corresponding class. Therefore, if a value of a required power is positioned at an end of the range of power of the corresponding class, the optical performance of the finished lens becomes lower relative to the most effective optical performance.
Although to produce the multifocal lenses using the stockpiled semifinished lens blanks improves productivity, the varieties of the semifinished lens blanks should be reduced so as to minimize inventories. This means that the varieties of the front surface of the lens (e.g., distribution of power on the lens surface, the number of combinations of addition power at the upper area of the lens and addition power at the lower area of the lens, the length of a progressive-power portion) are limited. Therefore, it is very difficult to stockpile semifinished spectacle lens blanks which cover all types of the customer's specifications.
The multifocal sprectacle lens disclosed in the above publication NO. SYO 62-30216 has progressive-power portions both in the upper area and the lower area of the lens. The multifocal spectacle lens in the publication is produced as follows.
Firstly, a shape of a back surface is selected from a spherical surface, a toric surface and a non-toric surface according to a customer's specification, for example, a required spherical power and a required cylindrical power. Then, the front surface having a progressive-power surface is designed, and is processed using a CNC (Computed Numerical Control).
Since the progressive-power surface of the multifocal lens shown in the publication is designed based on the selected shape of the back surface and the required power (i.e., the required spherical power and cylindrical power), it may be possible to obtain a lens having a sufficient optical performance. However, in this case, swinging and distortion of an image peculiar to mulitifocal spectacle lenses becomes greater in comparison with a multifocal spectacle lens having a progressive-power surface on its back surface.
In addition, since the front surface of the multifocal spectacle lens shown in the publication has three visual distance portions and two progressive-power portions, a shape of the front surface changes drastically. This makes the process for producing the lens difficult.